Differential cascode current mode driver

ABSTRACT

A current mode driver includes a tail current device, a differential pair of input transistors, cascode output transistors, and pre-charge circuits to charge cascode nodes between the differential pair of input transistors and the cascode output transistors. The current mode driver is driven by CMOS drivers that alternately turn the input transistors on and off. A wide-swing bias circuit provides bias voltages for the current mode driver. This bias voltage for the tail current device is closely matched to provide current matching between the bias circuit and the current mode driver.

FIELD

The present invention relates generally to current mode drivers, andmore specifically to scalable, low power, current mode drivers.

BACKGROUND

Integrated circuits typically have dedicated interface circuits tocommunicate with other integrated circuits and other systems. Somededicated interface circuits employ current mode drivers that drivecurrents between integrated circuits. Typical current mode driversinclude input transistors that do not completely turn off. Known methodsfor biasing these drivers includes the use of wide-swing bias circuits.Examples of wide-swing bias circuits are discussed in chapter six of:David A Johns & Ken Martin, “Analog Integrated Circuit Design,” (1997).

Signals that travel from one integrated circuit to another are becomingfaster and faster. As signal speeds increase, the effect of imperfect“channels” also increases. A “channel,” for the purposes of thisdescription, is any medium that the signal passes through. For example,a channel may consist of printed circuit board traces or wires routedbetween integrated circuits. One possible effect of an imperfect channelis frequency dependent attenuation of signal amplitudes. In general,when the signal amplitude is attenuated as a function of frequency, thesignal becomes smaller as the speed increases. When the signal gets toosmall, communications between integrated circuits can become unreliable.

One known method for compensating for frequency dependent attenuation isthe use of pre-equalization at the driver. Pre-equalization adjusts theamplitude of the driver depending on the frequency of outbound data.Successful pre-equalization compensates for signal loss in the channel,and results in a substantially constant amplitude received voltage wavefor low and high frequency data. One mechanism for pre-equalization isdescribed in: Ramin Faijad-Rad, Chih-Kong Ken Yang, Mark A. Horowitz,and Thomas H. Lee, “A 0.4-um CMOS 10-Gb/s 4-PAM Pre-Emphasis Serial LinkTransmitter,” Vol. 34, No. 5, IEEE Journal of Solid-State Circuits, (May1999).

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foralternate integrated circuit interfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a differential cascode current mode driver with awide-swing bias circuit;

FIG. 2 shows a differential cascode current mode driver;

FIG. 3 shows a wide-swing bias circuit;

FIG. 4 shows an integrated circuit with a simultaneous bidirectionalport circuit; and

FIG. 5 shows parallel differential cascode current mode drivers.

DESCRIPTION OF EMBODIMENTS

In the following detailed description of the embodiments, reference ismade to the accompanying drawings which show, by way of illustration,specific embodiments in which the invention may be practiced. In thedrawings, like numerals describe substantially similar componentsthroughout the several views. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. Other embodiments may be utilized and structural, logical,and electrical changes may be made without departing from the scope ofthe present invention. Moreover, it is to be understood that the variousembodiments of the invention, although different, are not necessarilymutually exclusive. For example, a particular feature, structure, orcharacteristic described in one embodiment may be included within otherembodiments. The following detailed description is, therefore, not to betaken in a limiting sense, and the scope of the present invention isdefined only by the appended claims, along with the full scope ofequivalents to which such claims are entitled.

FIG. 1 shows a differential cascode current mode driver with awide-swing bias circuit. Current driver 100 has a pair of differentialinput nodes 160 and 162, and a pair of differential output nodes 164 and166. In operation, a digital signal and its logical complement, “DATA”and “DATA#,” are provided on differential input nodes 162 and 160,respectively. In response to the digital input signal, a current appearson one of the two differential output nodes 164 and 166. For example,when the DATA signal is high, and the DATA# signal is low, a currentsignal “OUT” appears on node 164, and no current appears on node 166.When the input signals on differential input nodes 160 and 162 are inthe opposite state, no current appears on node 164, and a current signal“OUT#” appears on node 166.

Current mode driver 100 includes transistors 102, 104, 106, 108, 110,112, and 114. Transistor 102 is a “tail current” device that sourcescurrent from power supply node 101 to internal node 103. The amount ofcurrent sourced by transistor 102 is determined in part by a biasvoltage on node 105, shown as “BIAS1” in FIG. 1. This bias voltage, aswell as other bias voltages, are provided by wide-swing bias circuit300.

All of the transistors shown in FIG. 1, with the exception oftransistors 132 and 142, are P-type metal oxide semiconductor fieldeffect transistors (PMOSFETs), also referred to as PMOS transistors. Themethod and apparatus of the present invention is not limited to the useof PMOS transistors. For example, in some embodiments, N-type (NMOS)transistors are employed, and in others, bipolar junction transistors(BJT) are employed. One skilled in the art will appreciate that amultitude of embodiments exist, each having different types oftransistors and combinations of types of transistors. All of theseembodiments are within the scope of the present invention.

Transistors 104 and 106 form a differential input pair driven bydifferential data signals on nodes 116 and 118. In operation, thedifferential data signals on nodes 116 and 118 are generated bycomplementary metal oxide semiconductor (CMOS) drivers created fromtransistors 130, 132, 140, and 142. The differential data signals onnodes 116 and 118 transition substantially between power supply voltageson nodes 101 and 150. This is also referred to as “swinging rail torail.”

As nodes 116 and 118 transition in voltage, transistors 104 and 106alternately transition between an “off” state and an “on” state. Whenthe transistors are off, they do not conduct current from source todrain, and when on, they do conduct current from source to drain. Thecurrent sourced by tail current transistor 102 is, therefore, switchedbetween the two paths provided by the differential input pair as afunction of the input data signal.

Transistor 104 has a gate coupled to node 116, a source coupled tointernal node 103, and a drain coupled to cascode node 109. Likewise,transistor 106 has a gate coupled to node 118, a source coupled tointernal node 103, and a drain coupled to cascode node 111. As discussedabove, because of the switching action of the input differential pair,only one of cascode nodes 109 and 111 has a steady-state current flowingthereon at a time. For example, when the voltage on node 116 is high andthe voltage on 118 is low, the current from tail current transistor 102flows through input transistor 106 and on node 111, and input transistor104 is off and no current flows on node 109. Also for example, when thevoltage on node 116 is low and the voltage on node 118 is high, thecurrent from tail current transistor 102 flows through input transistor104 and on node 109, and input transistor 106 is off and no currentflows on node 111.

Current mode driver 100 also includes pre-charge transistors 108 and110. Pre-charge transistors 108 and 110 charge cascode nodes 109 and111, respectively, when no current flows on the respective cascode node.For example, when input transistor 104 is off and no current flows onnode 109, pre-charge transistor 108 is on and cascode node 109 chargesto a voltage value of “BIAS3.” Also for example, when input transistor106 is off, pre-charge transistor 110 is on and cascode node 111 ischarged to “BIAS3.” Pre-charge transistors 108 and 110 are examples ofprecharge circuits that pre-charge the cascode nodes when no currentflows thereon. In some embodiments, other pre-charge circuits are usedto charge the cascode nodes.

Current drive 100 also includes cascode output transistors 112 and 114.Cascode output transistor 112 is coupled from source to drain betweencascode node 109 and output node 166. Likewise, cascode transistor 114is coupled from source to drain between cascode node 111 and output node164. Casecode output transistors 112 and 114 are biased in saturation bya bias voltage “BIAS2” provided on node 115 by wide-swing bias circuit300.

The effective output capacitance of current driver 100 is small in partbecause cascode output transistors 112 and 114 operate in saturation,which provides a high impedance path to all of the parasiticcapacitances at the internal nodes of current driver 100. Current driver100 also has a high output impedance achieved by the cascodeconnections.

Because input transistors 104 and 106 have rail to rail input swings,they can be sized much smaller than cascode output transistors 112 and114. As a result, the gate capacitance on nodes 116 and 118 can be keptrelatively small, thereby reducing the dynamic power consumption of theCMOS drivers.

FIG. 2 shows a differential cascode current mode driver. Current driver200 includes transistor groups 202, 204, 206, 208, 210, 212, and 214.Transistor group 202 includes tail current devices biased at BIAS1 onnode 105. Transistor groups 204 and 206 form an input differential pair,and groups 208 and 210 form pre-charge circuits. Transistor groups 212and 214 form cascode output transistors. In general, for everytransistor in current mode driver 100 (FIG. 1), current mode driver 200includes a group of transistors coupled in parallel. As a result,current mode driver 200 has increased current drive on the output. Inthe embodiment shown in FIG. 2, current mode driver 200 has fourparallel transistors for every transistor in current mode driver 100,and has four times the output current drive. In other embodiments, morethan four transistors are employed, and the current drive is more thanfour times the current drive of current mode driver 100. In still otherembodiments, less than four transistors are employed, and the currentdrive is less than four times the current drive of current mode driver100.

In embodiments represented by FIG. 2, transistor groups 208 and 210 eachinclude four parallel transistors to form pre-charge circuits. In theseembodiments, internal capacitance values provided by the prechargecircuits are scaled the same as the current drive. In other embodiments,groups 208 and 210 include a different number of transistors than theremaining groups. For example, in some embodiments, groups 202, 204,206, 212, and 214 have four transistors as shown in FIG. 2, but groups208 and 210 each have a single transistor as shown in FIG. 1. Inembodiments with a single pre-charge transistor, the capacitance on thecascode nodes is reduced without compromising the increased currentdrive.

FIG. 3 shows a wide-swing bias circuit. Wide-swing bias circuit 300includes transistors 302, 304, 306, and 308. Transistors 302 and 304 arediode-connected. The term “diode-connected,” as used herein, refers to atransistor that has a gate and a drain coupled together, as dotransistors 302 and 304. Each of transistors 302 and 304 are in serieswith a current source, and a bias voltage is generated as the voltagedrops from the voltage on power supply node 101 across the diodeconnected transistor. The voltage drop across transistor 302 is used togenerate BIAS3 on node 113, and the voltage drop across transistor 304is used to generate BIAS2 on node 115.

Transistor 306 is diode-connected “around” transistor 308. The term“diode-connected around,” as used herein, describes a diode-connectedtransistor with another transistor coupled source to drain between thediode-connected transistor's drain and gate. The combination oftransistors 306 and 308 are in series with a current source, and thebias voltage BIAS1 on node 105 is generated by the voltage drop acrosstransistors 306 and 308.

The gate of transistor 308 has a fixed voltage applied thereto. Inembodiments represented by FIG. 3, the gate of transistor 308 is atground potential. Current 309 in wide-swing bias circuit 300 flowsthrough bias transistor 306 in series with transistor 308 having a gateat ground potential. Likewise, the tail current path in driver 100includes tail current transistor 102 in series with an input transistorhaving a gate at ground potential when one of the differential inputtransistors 104 or 106 is on, because the gate of the input transistoris substantially at ground potential because it is driven by a CMOSdriver. This configuration approximately matches the direct current (DC)operating points between driver 100 (FIG. 1) and bias circuit 300 so asto achieve a current match between the bias and the driver.

FIG. 4 shows an integrated circuit with a simultaneous bidirectionalport circuit. Integrated circuit 400 is shown with a simultaneousbidirectional port circuit coupled to conductors 430 and 432. In theembodiments represented by FIG. 4, conductors 430 and 432 aretransmission lines having a characteristic impedance of Z₀. Integratedcircuit 400 communicates with other integrated circuits (not shown)bidirectionally and simultaneously using both conductors 430 and 432. Insome embodiments, multiple simultaneous bidirectional ports exist withinintegrated circuit 400, and a bus is formed from multiple pairs ofconductors between integrated circuits. The simultaneous bidirectionalport circuit within integrated circuit 400, and the operation thereof,are explained below.

Integrated circuit 400 includes multiplexor 408, current mode outputdriver 412, current mode return driver 414, differential receiver 424,and termination resistors 416 and 418. Outbound data on node 402 is datagenerated within integrated circuit 400 that is to be transmittedthrough conductors 430 and 432 to be received by another integratedcircuit (not shown). Multiplexor 408 receives outbound data on node 402and pre-equalization data on node 406. Multiplexor 408 drives acombination of outbound data and pre-equalization data on nodes 410. Theamount of pre-equalization data driven on nodes 410 is selected byequalization select data on node 407. Delay match circuit 405 receivesoutbound data on node 402 and produces a delayed version on node 403.Delay match circuit 405 provides an amount of delay substantially equalto the delay of multiplexor 408.

Current mode output driver 412 receives outbound data on node 403, and acombination of outbound data and pre-equalization data on nodes 410, anddrives differential data lines 420. Differential data lines 420 exitintegrated circuit 400 at differential data nodes 440, and driveconductors 430 and 432. In some embodiments, differential data nodes 440are integrated circuit pads coupled to a package that includesintegrated circuit 400. Differential data lines 420 also feedback intointegrated circuit 400, and drive a differential input node ofdifferential receiver 420. Current mode return driver 414 drivesdifferential data lines 422. Differential data lines 422 do not drivenodes off integrated circuit 400, but do feedback into integratedcircuit 400 to drive a second differential input node of differentialreceiver 420.

Data on nodes 403 and 410 can drive current mode output driver 412 andcurrent mode return driver 414 to one of multiple logical states.Drivers 412 and 414 are current drivers that include a number of smallercurrent drivers configured in parallel. Example embodiments of drivers412 and 414 are shown in more detail in FIG. 5. Current mode outputdriver 412 switches current between differential data lines 420 as afunction of the logical state of data on nodes 403 and 410. Likewise,current mode return driver 414 switches current between differentialdata lines 422 as a function of the same logical states.

As previously described, differential receiver 424 has two sets ofdifferential input nodes, one coupled to differential data lines 420,and the other coupled to differential data lines 422. Differential datalines 420 include data driven by both integrated circuit 400 and anyother integrated circuits coupled to conductors 430 and 432. Incontrast, data lines 422 only include data driven by integrated circuit400. Differential receiver 424 subtracts the differential voltage ondifferential data lines 422 from a differential voltage on differentialdata lines 420 to produce inbound data on node 426. Inbound data on node426 represents the outbound data sent from a different integratedcircuit coupled to integrated circuit 400 across the simultaneousbidirectional interface.

In some embodiments, multiplexor 408 provides drivers 412 and 414 withboth outbound data and pre-equalization data. In these embodiments,nodes 410 include multiple physical nodes. For example, in someembodiments, multiplexor 408 drives a replica of the outbound data aswell as pre-equalization data on multiple physical nodes to drivers 124and 126. Pre-equalization data is utilized within driver 412 to adjustthe amplitude of the output current drive on bidirectional data lines420 to compensate for channel variations in conductors 430 and 432. Forexample, if high frequency signals are attenuated in conductors 430 and432, current mode output driver 412 can utilize pre-equalization data todrive a higher amplitude when outbound data changes at a higherfrequency.

In some embodiments, the simultaneous bidirectional port circuit withinintegrated circuit 400 performs variable equalization. In theseembodiments, equalization select data on node 407 sets the amount ofequalization provided by multiplexor 408. Multiplexor 408 can provideall outbound data, all pre-equalization data, or a mix of the two.

As previously described, drivers 412 and 414 are current mode driversthat switch currents between output nodes as a function of the logicalstate of the input nodes. Current mode output driver 412 drives adifferential current on differential data lines 420. This differentialcurrent is terminated by the characteristic impedance (Z₀) of conductors430 and 432, and the resistance (R₁) of resistors 416. Therefore,current mode output driver 412 is terminated with an impedance equal tothe parallel combination of Z₀ and R₁. In contrast, current mode returndriver 414 drives differential data lines 422 which are terminated onlyby resistors 418 having a resistance value of R₂.

In some embodiments, the resistance values of resistors 416 and 418 aremodified in combination with the current drive of drivers 412 and 414 toprovide voltage scaling at the input to differential receiver 424. Forexample, in some embodiments, R₂ is a higher resistance value than theparallel combination of R₁ and Z₀, thereby allowing less current driveto be provided by current mode return driver 414 while maintaining theproper voltage level for comparison at differential receiver 424.

Integrated circuit 400 can be any type of integrated circuits capable ofincluding simultaneous bidirectional port circuits. For example,integrated circuit 400 can be a processor such as a microprocessor, adigital signal processor, a microcontroller, or the like. Integratedcircuit 400 can also be an integrated circuit other than a processorsuch as an application-specific integrated circuit (ASIC), acommunications device, a memory controller, or a memory such as adynamic random access memory (DRAM).

FIG. 5 shows parallel differential cascode current mode drivers. Currentmode output driver 412 includes 12 parallel drivers 200. Drivers 200 areshown in more detail in FIG. 2. Current mode return driver 414 includes12 parallel drivers 100. Drivers 100 are shown in more detail in FIG. 1.In embodiments represented by FIG. 5, seven of the 12 drivers 200 incurrent mode output driver 412 are driven by outbound data on node 403,and five of the 12 drivers 200 are driven by data from multiplexor 408on nodes 410. Likewise, seven of the 12 drivers 100 in current modeoutput driver 414 are driven by outbound data on node 403, and five ofthe 12 drivers 100 are driven by data from multiplexor 408 on nodes 410.

In the embodiments of FIG. 5, seven drivers in each of current modedrivers 412 and 414 always drive outbound data on differential datalines 420 and 422. Five drivers in each of current mode drivers 412 and414, on the other hand, can drive either outbound data orpre-equalization data depending on the state of the equalization selectdata on node 407 (FIG. 4). Because multiplexor 408 can drive a variablenumber of drivers with pre-equalization data, variable pre-equalizationcan be achieved. In the embodiment of FIG. 5, six levels ofpre-equalization can be provided by driving between zero and fivedrivers 100 and 200 with pre-equalization data. The six levels provide 0dB, 1.5 dB, 3.5 dB, 6 dB, 9.5 dB, and 16 dB of pre-equalization. Ingeneral, current mode output drivers 412 and 414 can include any numberof drivers 200 and 100, respectively, to provide an range and resolutionof pre-equalization.

Each driver 100 has an output current drive capability equal to ¼ thedrive of each driver 200. As a result, current mode output driver 412has a current drive equal to four times that of current mode returndriver 414. Accurate current scaling between drivers 412 and 414 can beachieved by varying the number of transistors in the groups of driver200 (FIG. 2).

It is to be understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reading and understanding theabove description. The scope of the invention should, therefore, bedetermined with reference to the appended claims, along with the fullscope of equivalents to which such claims are entitled.

What is claimed is:
 1. A current mode driver circuit comprising: a pairof differential output nodes; a cascode output circuit coupled betweenthe pair of differential output nodes and a pair of internal cascodenodes; a pair of pre-charge circuits to pre-charge the pair of cascodenodes; a differential pair of input transistors coupled to the pair ofinternal cascode nodes, the differential pair of input transistorshaving control nodes coupled to a differential pair of input signalnodes, wherein the pair of pre-charge circuits includes transistorshaving control nodes coupled to the differential pair of input signalnodes; a tail current transistor coupled between a power supply node andthe differential pair of input transistors; and a bias circuit to biasthe tail current transistor, the bias circuit including a firsttransistor with a gate at a fixed potential, and a second transistordiode connected around the first transistor.
 2. The current mode drivercircuit of claim 1 wherein each of the pair of pre-charge circuitsincludes a transistor to pre-charge the corresponding internal cascodenode when current is not flowing.
 3. The current mode driver of claim 1further comprising CMOS drivers coupled to the control nodes of thedifferential pair of input transistors.
 4. The current mode driver ofclaim 1 wherein the cascode output circuit includes a plurality ofcascode transistors in parallel between an internal cascode node and anoutput node.
 5. The current mode driver of claim 4 further comprising afirst plurality of parallel connected transistors coupled in parallelwith a first of the differential pair of input transistors, and a secondplurality of parallel connected transistors coupled in parallel with asecond of the differential pair of input transistors.
 6. An integratedcircuit comprising: a differential receiver having first and seconddifferential input node pairs; a first current mode driver having aninput node coupled to an outbound data node, and a differential outputnode pair coupled to the first differential input node pair of thedifferential receiver; and a second current mode driver having an inputnode coupled to the outbound data node, a differential output node paircoupled to the second differential input node pair of the differentialreceiver, cascode output transistors coupled between the differentialoutput node and a pair of cascode nodes, a pair of pre-charge circuitsto pre-charge the pair of cascode nodes, a pair of differential inputtransistors coupled between the outbound data node and the pair ofcascode nodes, and a tail current device coupled between an uppervoltage supply node and the pair of differential input transistors;wherein the first current mode driver includes a plurality of parallelconnected transistors for every transistor in the second current modedriver.
 7. The integrated circuit of claim 6 wherein the integratedcircuit includes at least one integrated circuit pad, and the firstcurrent mode driver differential output node pair is coupled to the atleast one integrated circuit pad.
 8. The integrated circuit of claim 6further including CMOS drivers coupled to the outbound data node todrive data to both the first and second current mode drivers.
 9. Theintegrated circuit of claim 6 further comprising a wide-swing biascircuit to bias the tail current device.
 10. The integrated circuit ofclaim 9 wherein the wide-swing bias circuit comprises: a firsttransistor with a gate coupled to a fixed voltage node; and a secondtransistor diode connected around the first transistor, the secondtransistor having a gate coupled to the tail current device.
 11. Acurrent mode driver comprising: a pair of differential input nodes and apair of differential output nodes; a tail current transistor coupledbetween a voltage supply node and a first node, the tail currenttransistor having a gate coupled to a first bias node; a differentialpair of input transistors coupled between the first node and a pair ofcascode nodes, the differential pair of input transistors having gatescoupled to the pair of differential input nodes; a pair of cascodeoutput transistors coupled between the pair of cascode nodes and thepair of differential output nodes, the pair of cascode outputtransistors having gates coupled to a second bias node; a pair ofpre-charge circuits to pre-charge the pair of cascode nodes to a voltageon a third bias node; and a wide-swing bias circuit to produce biasvoltages on the first, second, and third bias nodes, wherein thewide-swing bias circuit comprises a first transistor having a gatecoupled to a fixed voltage, and a second transistor diode-connectedaround the first transistor, the second transistor having a gate coupledto the first bias node.
 12. The current mode driver of claim 11 whereinthe pre-charge circuits each comprise a transistor having a source anddrain coupled between a corresponding cascode node and the third biasnode, and having a gate coupled to one of the differential pair of inputnodes.
 13. The current mode driver of claim 12 further comprising a pairof CMOS drivers to drive the differential pair of input nodes.
 14. Anintegrated circuit comprising: a differential receiver having first andsecond differential input node pairs; a first current mode driver havingan input node coupled to an outbound data node, and a differentialoutput node pair coupled to the first differential input node pair ofthe differential receiver; and a second current mode driver having aninput node coupled to the outbound data node, a differential output nodepair coupled to the second differential input node pair of thedifferential receiver, cascode output transistors coupled between thedifferential output node and a pair of cascode nodes, a pair ofpre-charge circuits to pre-charge the pair of cascode nodes, a pair ofdifferential input transistors coupled between the outbound data nodeand the pair of cascode nodes, and a tail current device coupled betweenan upper voltage supply node and the pair of differential inputtransistors.
 15. The integrated circuit of claim 14 wherein theintegrated circuit includes at least one integrated circuit pad, and thefirst current mode driver differential output node pair is coupled tothe at least one integrated circuit pad.
 16. The integrated circuit ofclaim 14 further including CMOS drivers coupled to the outbound datanode to drive data to both the first and second current mode drivers.17. The integrated circuit of claim 14 further comprising a wide-swingbias circuit to bias the tail current device.
 18. The integrated circuitof claim 17 wherein the wide-swing bias circuit comprises: a firsttransistor with a gate coupled to a fixed voltage node; and a secondtransistor diode connected around the first transistor, the secondtransistor having a gate coupled to the tail current device.